This invention relates generally to EMI filter terminal pin subassemblies and related methods of construction, particularly of the type used in implantable medical devices such as cardiac pacemakers and the like, to decouple and shield undesirable electromagnetic interference (EMI) signals from the device. More specifically, the present invention relates to a ceramic chip capacitor EMI filter assembly and related installation method, including utilization of groupings of a plurality of chip filter capacitors which vary in capacitance value.
Implantable medical devices employ a wide variety of leads which are placed in body tissue or fluids. These include but are not limited to atrial/ventricle unipolar, atrial/ventricle bipolar, subcutaneous patch, transthoracic impedance monitor, telemetry, cochlear implants and the like. In some implantable defibrillator designs the metallic case or housing of the device acts as one electrode or lead. Leads which are closely spaced together in the body (such as bipolar) tend to pick up EMI signals which are in phase (common mode EMI). Lead schemes which have substantial separation between the leads and are close to an emitter (such as a cellular phone) tend to pick up EMI signals which are not in phase (differential mode EMI). In addition, certain implantable medical device sensing circuits are highly sensitive (such as unipolar atrial blood pool sensing leads) which tends to make them more susceptible to EMI. Other implantable device circuits may be limited in the amount of capacitance to ground that they can tolerate (certain defibrillator high voltage outputs or transthoracic impedance monitors, for example).
Feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators or the like, the hermetic terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the medical device.
There are two primary ceramic capacitor geometries in common use in the industry--the rectangular chip and the feedthrough (often called a discoidal capacitor). The ceramic monolithic rectangular chip capacitor (or "chip capacitor") is produced in very high commercial volumes in highly automated facilities. Over the years the cost of ceramic chip capacitors has dropped a great deal. It is now common to purchase certain value chip capacitors for only a few pennies. The ceramic feedthrough capacitor is only produced in a small fraction of the chip capacitor volume. Accordingly, feedthrough capacitor production has not been nearly as automated. In addition, the feedthrough capacitor is inherently more expensive to produce due to drilling and centering the through hole, tighter dimensional control, reduced volumetric efficiency and difficulty in automating the manufacturing process. Typically the cost of a particular value chip capacitor is ten to twenty percent of the cost of an equivalent value discoidal feedthrough capacitor. Because of the trend in the medical device industry to reduce cost, it would be highly desirable to use relatively inexpensive ceramic chip capacitors in place of feedthrough capacitors, where practicable.
Previously, the feedthrough capacitor-type has been the capacitor of choice for use in high performance or broadband EMI filters because it provides effective attenuation over very wide frequency ranges. A serious shortcoming of the rectangular chip capacitor is that its equivalent circuit model consists of a capacitor in series with a lumped inductor (the inductance comes from its leads, internal electrode plates and connection wiring). This means that all chip capacitors will self resonate when their inductive reactance and capacitive reactance become equal. The frequency in MHz at which this occurs is defined as the "self resonant frequency" or SRF, which is determined in accordance with the formula: ##EQU1## wherein, L=Inductance in microhenries, and
C=Capacitance in microfarads.
Above the SRF, the chip capacitor becomes increasingly inductive and ceases to be an effective EMI filter. The unique geometry of the feedthrough capacitor, however, eliminates this lumped series inductance. Accordingly, a properly designed and installed feedthrough capacitor does not have a SRF above which it becomes an inductor (feedthrough capacitors do exhibit a minor self resonance, but continue to perform as an effective capacitor bypass element above this SRF).
Two different general approaches are commonly used to eliminate or reduce stray or unwanted EMI signals. One very effective but relatively costly approach is where the hermetic terminal pin assembly has been combined directly with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device. In a typical unipolar construction as shown in U.S. Pat. No. 5,333,095, a coaxial ceramic feedthrough filter capacitor is used in a feedthrough assembly to suppress and decouple undesired interference or noise transmission along a terminal pin. The feedthrough filter capacitor comprises a so-called discoidal capacitor having two sets of electrode plates embedded in spaced relation within an insulative dielectric substrate or base, formed typically as a ceramic monolithic structure. One set of the electrode plates is electrically connected at an inner diameter cylindrical surface of the discoidal capacitor structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets varies in accordance with the capacitance value and the voltage rating of the discoidal capacitor. The outer feedthrough capacitor electrode plate sets (or "ground" plates) are coupled in parallel together by a metalized band which is, in turn, coupled to the ferrule by conductive adhesive, soldering, brazing, or the like. The inner feedthrough capacitor electrode plate sets (or "active" plates) are coupled in parallel together by a metalized band which is, in turn, coupled to the lead wire(s) by conductive adhesive, soldering, brazing, or the like. In operation, the feedthrough capacitor permits passage of relatively low frequency electrical signals along the terminal pin while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (6) and additional lead configurations. The feedthrough capacitors of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal, such as titanium alloy which is electrically coupled to the feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function. Also see for example, the feedthrough capacitor subassemblies disclosed in U.S. Pat. Nos. 3,920,888; 4,152,540; 4,421,947; 4,424,551 and 5,333,095.
A second type of EMI filter approach in common use in implantable medical devices involves installation of ceramic chip capacitors on the circuit board, substrate or flex cables leading to the hermetic feedthrough terminal. For example, it is known to locate an EMI low pass filter and related chip capacitors on the circuit board or substrate of an implantable medical device, or to mount a chip capacitor onto a circuit flex cable near the hermetic terminal of a pacemaker. These are very cost effective methods, but they do not make a very effective EMI filter, particularly over broad frequency ranges. Both of these approaches provide ineffective EMI filters at high frequencies due to the parasitic resonance and coupling caused by the substantial inductance and capacitance of the circuit traces and connecting wires. The physical separation of the chip capacitors from the point of penetration into the shield housing (the hermetic seal terminal) creates excessive loop inductance and allows the unwanted signals to penetrate to the interior of the shield. Once the EMI is inside, it is very difficult to control as it will tend to couple across filtering elements to sensitive circuitry.
Accordingly, there is a need for an EMI filter that utilizes cost effective ceramic chip capacitors in a manner to achieve the beneficial EMI filtering characteristics similar to feedthrough filter capacitors. Such a unique EMI filter should utilize chip capacitors that are mounted in groups which vary in physical size, dielectric material and capacitance value so that they self-resonate at different frequencies to provide the filter desirable broadband frequency attenuation. Morever, a novel method of chip capacitor geometry/termination-metallization is needed which significantly reduces the internal inductance of the chip capacitor to improve its high frequency performance as an EMI filter. The present invention fulfills these needs and provides other related advantages.